Tuners for television receivers include tunable filters in the RF amplifier and local oscillator sections. Typically, a tunable filter includes a varactor diode exhibiting a variable capacitance controlled in response to a tuning voltage, two or more inductance elements, and at least one bandswitching diode connected to the inductance elements for selecting the inductance configuration of the tunable circuit in response to a bandswitching signal.
Additional reactive elements may be associated with the tunable filter for various reasons. For example, elements may be provided to produce trap or other filter response for rejecting or inhibiting unwanted signals. Elements may also be provided to make the frequency selective response of a tunable filter in one part of the tuner track the frequency response of another tunable filter in another part of the tuner.
Sometimes it is desirable that the additional reactance elements be functionally connected in one band but not in another band. For example, it may be desirable to have a trap active for one band but inactive in another band in which it may interfere with the desired function of the associated tunable filter. Of course, a second switching diode responsive to the bandswitching signal (in addition to the bandswitching diode used to select the inductance configuration) may be provided for the latter purpose.
It is desirable that extra reactance and switching elements be kept to a minimum for cost and reliability purposes. This goal can be accomplished by making dual use of at least some existing elements of the tunable circuit itself.
U.S. Pat. No. 4,023,106 issued in the name of Keisake Utsunomiya, discloses an example of such an arrangement. In the disclosed arrangement, an additional capacitor is provided to form a trap with an inductor otherwise used for impedance matching in a first band. A bandswitching diode functionally disconnects both the capacitor and the inductor in a second band. Thus the inductor is not available for use in the second band.